This invention relates to a sample assembly for a thermoelectric analyzer which can measure an electric property of a sample as the sample temperature varies. Means for measuring an electric property of a sample during sample temperature variation includes typically a TSC (Thermally Stimulated Current) method and other measurement means such as DEA (Dielectric Analysis: thermal relaxation measurement), DLTS (Deep Level Transient Spectroscopy), ICTS (Isothermal Capacitance Transient Spectroscopy), TSIC (Thermally Stimulated Ionic Current), IV (Current-Voltage characteristic) and CV (Capacitance-Voltage characteristic). This invention relates to a sample assembly for a thermoelectric analyzer which can perform any one of these measurement methods.
The TSC method is one of the traditional methods known in the field of thermal analysis and can measure a current occurring in the sample as the sample temperature varies, the result of measurement being analyzed in various ways. It is known that the crystal defect in a sample can be analyzed using the TSC method. For example, in Japanese Journal of Applied Physics, Vol. 27, No. 2, 1988, pp.260-268 (referred to hereinafter as the first publication), a semi-insulating GaAs (gallium arsenite) sample was so analyzed that a TSC spectrum was measured in a low temperature range from the liquid helium temperature to the room temperature to analyze the deep level traps. Further, in Rev. Sci. Instrum., Vol. 63, No. 12, 1992, p.5714-5725 (referred to hereinafter as the second publication), a SiO2 (silicon dioxide) layer of a MOS capacitor was so analyzed that a TSC spectrum was measured in a temperature range from the room temperature to 300xc2x0 C. to analyze the density of positive holes or electrons.
In these publications, there was the following disclosure regarding a sample assembly. In the first publication, the GaAs sample has the top surface which has a central region covered with a semitransparent aluminum electrode (which becomes one of the electrodes). The top surface of the sample has also a periphery on which a guard ring is formed. On the other hand, the sample has the back surface which is covered with a thick aluminum film (which becomes the other of the electrodes). It is noted that, in the figure of the first publication, although the sample is mounted on a support which can be heated, it is not clear in what manner the sample has been mounted on the support.
In the second publication, a sample is mounted on an insulating substrate. The substrate has both ends to which stainless steel pins are fixed along with aluminum washers. An aluminum wire is connected between the washer at one end of the substrate and the back electrode of the sample (SiO2 capacitor). The sample, having the back side to which the wire has been connected, is fixed at its back electrode to the center of the substrate by silver past including epoxy resin. Besides, the sample has, on its top surface, a gate which is connected with the other washer at the other end of the substrate via another aluminum wire.
As to the sample assembly disclosed in the first publication, there is no disclosure regarding a specific manner for fixing the sample to the substrate, therefore, its bonding quality can not be judged. As to the sample assembly disclosed in the second publication, the silver paste is used for bonding the sample to the substrate, raising a problem of poor temperature uniformity in the sample. Further, since the aluminum washers and aluminum wires are used and the electric circuit, from the sample to the stainless steel pins at the both ends of the substrate, is not mirror-symmetrical, a contact-electromotive force may become large and a thermoelectromotive force may occur during the temperature increase or decrease, resulting in measurement noise.
It is an object of the invention to provide a sample assembly, for a thermoelectric analyzer, having good temperature uniformity and generating a small contact-electromotive force and a small thermoelectromotive force.
A sample assembly for a thermoelectric analyzer according to the invention includes an electrically-insulating substrate to which a sample is fixed via an adhesive layer. The material of the adhesive layer is indium or gold-tin alloy. The substrate has a pair of junction electrode layers formed thereon. The sample has a pair of electrode layers formed on the same plane of the sample. One of the electrode layers is connected with one of the junction electrode layers by an electrically-conductive wire, while the other of the electrode layers is connected with the other of the junction electrode layers by another electrically-conductive wire.
The indium used for the adhesive layer has high thermal conductivity, resulting in good heat conduction between the sample and the substrate and therefore good temperature uniformity in the sample. Besides, the indium is soft metal, so that it solves the below-described problem. Under the condition of wide-range temperature variation (for example, between the liquid nitrogen temperature and the room temperature), a thermal stress may occur between the sample and the substrate because of different rates of thermal expansion. The indium can absorb the thermal stress so that internal strain hardly occurs in the sample. The indium may be replaced by gold-tin alloy, for example, 88% Au-12% Sn, which also has good thermal conductivity.
The substrate is made of preferably a highly electrically-insulating and highly thermally-conductive material which may be aluminum nitride (AlN), boron nitride (BN), beryllium oxide (BeO) or aluminum oxide (Al2O3).
The sample assembly is adapted to be supported by two support rods. Gold washers are preferably inserted between the support rods and the junction electrode layers, decreasing the influence, on the substrate, of the thermal displacement of the support rods.
Each of the electrode layers on the sample and the junction electrode layers on the substrate may be made of a multilayer including the top layer which is preferably a gold layer. In that case, gold wires are bonded to the gold layers. The bonding between two gold parts decreases a contact-electromotive force and a thermoelectromotive force which may occur between the wires and the electrode layers and between the wires and the junction electrode layers. Furthermore, the pair of electrode layers on the sample and the pair of junction electrode layers on the substrate may be arranged mirror-symmetrical with respect to the center of the sample, so that the contact-electromotive force and the thermoelectromotive force mentioned above can be cancelled even when such forces occur.
The sample assembly may be applied to typically a TSC (Thermally Stimulated Current) analyzer or any other thermoelectric analyzer which can perform thermoelectric analysis such as DEA (Dielectric Analysis: thermal relaxation measurement), DLTS (Deep Level Transient Spectroscopy), ICTS (Isothermal Capacitance Transient Spectroscopy) TSIC (Thermally Stimulated Ionic Current), IV (Current-Voltage characteristic) or CV (Capacitance-Voltage characteristic). The sample may be preferably compound semiconductor such as GaAs.
The sample assembly for a thermoelectric analyzer according to the invention has the following advantages. The adhesive layer made of indium or gold-tin alloy is used to bond the sample with the substrate, resulting in good temperature uniformity in the sample. The gold washers are inserted between the support rods and the junction electrode layers on the substrate, decreasing the influence of the thermal displacement of the support rods. Each of the electrode layers on the sample and the junction electrode layers on the substrate includes the top layer which is a gold layer and the gold wires are bonded to the gold layers, resulting in a small contact-electromotive force and a small thermoelectromotive force. The pair of the electrode layers on the sample and the pair of the junction electrode layers on the substrate are arranged mirror-symmetrical, so that the contact-electromotive force and the thermoelectromotive force mentioned above can be cancelled even when such forces occur.